Method for operating a power tool

ABSTRACT

The present invention is based on a method for operating a power tool with an energy accumulator ( 12 ), in particular a rechargeable energy accumulator, which supplies power to an electric drive motor ( 10 ).  
     It is provided that a clock frequency is generated by an electronic unit ( 15 ), with which a gate (X 4 ) of a MOSFET ( 17 )—which supplies operating voltage to the drive motor ( 10 )—is switched on with each cycle, a switching-off of the MOSFET ( 17 ) being carried out within one cycle using different signals, as a function of operating parameters.

BACKGROUND INFORMATION

The present invention is directed to a method for operating a power toolaccording to the definition of the species in Claim 1.

When rechargeable energy accumulators are used with power tools, totaldischarge of the energy accumulator must be avoided, since this cancause the energy accumulator to become irreversibly damaged, or, at theleast, its service life can be greatly reduced.

ADVANTAGES OF THE INVENTION

The method according to the present invention for operating a power toolwith an energy accumulator is suited for use, in particular, with powertools with lithium ion batteries. The energy accumulator is successfullyprotected, and good regulating properties for the power tool areattained. A clock frequency is generated by an electronic unit, withwhich a gate of a MOSFET, which supplies operating voltage to the drivemotor, is switched on with each cycle, it being possible to switch offthe MOSFET within one cycle using different signals, as a function ofoperating parameters. When the MOSFET is switched off, the first resultis not that the drive motor is shut off, but rather that a pulse widthof the drive motor is reduced. The motor current and, therefore, thetorque remain high or even at a maximum level, and only the rotationalspeed decreases.

Preferably, when a specified threshold value of an operating parameterof the power tool is reached, a pulse width of triggering signals of thedrive motor is reduced. The power tool merely reduces the pulse width,but does not shut off completely.

In a favorable embodiment, when a maximum permissible threshold of anelectric current from the energy accumulator is reached, the pulse widthis reduced.

In a further favorable embodiment, when a maximum permissible power lossof the energy accumulator and/or the drive motor is reached, the pulsewidth is reduced. Resetting the gates does not result in the power toolbeing switched off. Instead, it only results in a reduction in therotational speed. Advantageously, the power loss is determined bydetermining a signal in proportion to ohmic losses in the energyaccumulator and/or the drive motor, and a power loss that is abovepermissible losses is integrated. To determine the power loss, thecurrent from the energy accumulator can be determined based on acurrent-proportional signal and converted to a signal that isproportional to the square of the current, a difference between thecurrent value and the current threshold can be calculated, and thedifference can be integrated with respect to time. The power loss thatis above the permissible losses is therefore integrated.

In a favorable embodiment, when a maximum permissible temperature of theenergy accumulator is reached, a measured current signal is modifiedsuch that the drive motor is gently turned off. By intentionally andspecifically adulterating the current signals, an uncomfortable, abruptshut-off can be prevented.

In a favorable embodiment, when a permissible minimum electrical voltageof the energy accumulator is fallen below, the gate is shut off.Preferably, the drive motor is not shut off completely until the energyaccumulator is discharged.

In a favorable embodiment, drilling-site illumination of the power toolis not switched off until the drive motor is shut off completely. Thepower tool operator can continue working nearly interference-free evenwhen threshold values are exceeded or fallen below, while the energyaccumulator is reliably protected against harmful total discharge.

It is advantageously provided that the pulse width of the gate voltageis adjusted in an on-off switch of the power tool using a potentiometer.Using a logarithmic curve of the pulse width as a function of adisplacement path of the on-off switch, the rotational speed of thepower tool can be adjusted very precisely between 0 and 100%.

DRAWING

Further embodiments, aspects and advantages of the present inventionalso result independently of their wording in the claims, withoutlimitation to generality, from an exemplary embodiment of the presentinvention presented below with reference to the drawing.

FIG. 1 shows a schematic diagram of a preferred power tool;

FIG. 2 shows a sequence of functions of a preferred method; and

FIG. 3 shows an example of an overall schematic diagram of a preferredelectronic unit.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

FIG. 1 shows a schematic diagram of a preferred power tool. Anelectronic unit 15 controls gate impulses of a MOSFET 17 acting as aswitch, which supplies operating voltage from an energy accumulator 12to a switch 11 and, therefore, a drive motor 10. FIG. 3 shows, as anexample, an overall schematic diagram of electronic unit 15. Theinterplay of individual functional blocks of the method according to thepresent invention is depicted in FIG. 2.

Switch 11 switches the direction of rotation of drive motor 10 betweenright and left rotation as desired. In a free-wheeling phase, the motorcurrent flows through a free-wheeling diode 18. Gate X4 of MOSFET 17 canbe applied to switch 11. A measuring resistor 19 is connected betweensource X2 of MOSFET 17 and negative terminal X1 of energy accumulator12, as shown in FIG. 3, with which an electrical current flow fromenergy accumulator 12 is measured.

A temperature of energy accumulator 12 is determined using a sensor 16and transmitted to temperature input X8. A capacitor 14 is locatedparallel to energy accumulator 12. An on-off switch 13 turns the supplyvoltage to drive motor 10 and, via input X5, to electronic unit 15 on oroff.

These functional blocks protect the energy accumulator, which ispreferably designed as a lithium ion battery, and provide the power toolwith good regulating properties.

The electronic unit 15 generates, in voltage block 20, a clock frequencyof a few kHz, preferably between 7 and 10 kHz, and particularlypreferably 8 kHz. At the beginning of each cycle, a gate voltage in gatevoltage block 21 is applied to input S, which therefore turns on MOSFET17. MOSFET 17 can be turned off via resetting at input R during a cycleusing various signals.

The electric current from energy accumulator 12 flows through measuringresistor 19 and generates a current-proportional signal, which isamplified. When a maximum value is reached, in particular a maximumpermissible discharge current of energy accumulator 12, e.g., approx. 30A when a lithium ion battery is used, the gate voltage in gate voltageblock 21 is reset. As a result, the power tool merely reduces its pulsewidth, but does not shut off completely. The motor current and,therefore, the torque remain at a maximum level, and only the rotationalspeed decreases.

Currrent-proportional signal I is determined in current measurementblock 23 and subsequently converted in integration block 24 to a signalleft, which is proportional to the square of the current. A limitI_(grenz) is subtracted from this new signal I_(eff). This differenceI_(eff)−I_(grenz) is integrated with respect to time. Signal I_(eff) isproportional to the ohmic losses (I²·R) in energy accumulator 12 anddrive motor 10. That is, the power loss above the permissible losses isintegrated. When this integral reaches a limit, the gate voltage in gatevoltage block 21 is reset. This does not cause the power tool to shutoff, either. It only reduces the rotational speed.

Voltage U_(bat) of energy accumulator 12 is monitored continually infunctional block 25, to protect lithium ion batteries against totaldischarge. Lithium ion batteries are highly susceptible to totaldischarge. If the actual value falls below limit U_(batt,min), which isadjusted as a function of load (I·R compensation), electronic unit 15turns off gate X4, i.e., the gate voltage is rest in gate voltage block21. To perform I·R compensation, functional block 25 receives a signalfrom current measurement block 23. This initially results in a reducedrotational speed. If energy accumulator 12 is discharged, the power toolshuts off completely. In this case, drilling-site illumination 28 infunctional block 26 is switched off.

When energy accumulator 12 is designed as a lithium ion battery, as isknown, e.g., with nickel-based cells, it must be protected againstexcessive temperatures. The temperature in energy accumulator 12 isdetermined (functional block 27) using a temperature resistor16—preferably a NTC resistor—which is preferably integrated in energyaccumulator 12. If a specified limit is exceeded, electronic unit 15adulterates the current signals in functional block 24 such that thepower tool is shut off gently and not abruptly.

Finally, the pulse width of the gate voltage can be adjusted using apotentiometer in on-off switch 13 (pulse-width block 22). Using alogarithmic dependence between a displacement path of the potentiometerand the pulse width, the rotational speed of the power tool can beadjusted very precisely between 0 and 100%.

Particularly preferably, the power tool is a cordless screwdriver with alithium ion battery.

1. A method for operating a power tool with an energy accumulator (12),in particular a rechargeable energy accumulator, which supplies power toan electric drive motor (10), wherein a clock frequency is generated byan electronic unit (15), with which a gate (X4) of a MOSFET (17), whichsupplies operating voltage to the drive motor (10), is switched on witheach cycle, a switching-off of the MOSFET (17) being carried out withinone cycle by different signals, as a function of operating parameters.2. The method as recited in claim 1, wherein, when a specified thresholdvalue of an operating parameter of the power tool is reached, a pulsewidth of triggering signals of the drive motor (10) is reduced.
 3. Themethod as recited in claim 1 or 2, wherein, when a maximum permissibleelectric current threshold of an electric current from the energyaccumulator (12) is reached, the pulse width is reduced.
 4. The methodas recited in one of the preceding claims, wherein, when a maximumpermissible power loss of the energy accumulator (12) and/or the drivemotor (10) is reached, the pulse width is reduced.
 5. The method asrecited in claim 4, wherein a signal is determined that is proportionalto the ohmic losses in the energy accumulator (12) and/or the drivemotor (10), and a power loss that is above permissible losses isintegrated.
 6. The method as recited in claim 5, wherein the currentfrom the energy accumulator (12) is obtained from a current-proportionalsignal, converted to a signal that is proportional to the square of thecurrent, a difference between the current value and the currentthreshold is calculated, and the difference is integrated with respectto time.
 7. The method as recited in one of the preceding claims,wherein, when a maximum permissible temperature of the energyaccumulator (12) is reached, a measured current signal is modified suchthat the drive motor (10) is gently turned off.
 8. The method as recitedin one of the preceding claims, wherein, when a minimum permissibleelectrical voltage of the energy accumulator (12) is reached, the gate(X4) is switched off.
 9. The method as recited in claim 8, wherein, whenthe energy accumulator (12) is discharged, the drive motor (10) isswitched off completely.
 10. The method as recited in one of thepreceding claims, wherein drilling-site illumination is not switched offuntil the drive motor (10) is shut off completely.
 11. A power tool withan energy accumulator (12), in particular a rechargeable energyaccumulator, which supplies power to an electric drive motor (10),wherein it is possible to generate a clock frequency using an electronicunit (15), with which a gate (X4) of a MOSFET (17)—which suppliesoperating voltage to the drive motor (10) is capable of being switchedon with each cycle, it being possible to switch off the MOSFET (17)within one cycle using different signals, as a function of operatingparameters.